Since polycarbonate is excellent in heat resistance, impact resistance and transparency, it has been widely used in many fields in recent years. Various studies have been carried out with processes for production of polycarbonate. Among them, polycarbonate derived from aromatic dihydroxy compounds such as 2,2-bis(4-hydroxyphenyl)propane, hereinafter “bisphenol A”, is industrialized by both processes of interfacial polymerization and melt polymerization.
According to the interfacial polymerization, polycarbonate is produced from bisphenol A, an aromatic monohydroxy compound such as p-tert-butyl phenol and phosgene.
Since it can be produced under the condition of relatively low temperature, polycarbonate thus obtained is usually a straight-chain polymer which exhibits a Newtonian property when it is melted. That is, with regard to shearing flowability, its shearing rate dependency of melt viscosity is small, and with regard to elongation flowability, it exhibits an extremely low viscosity. Therefore, when conducting large-scale extrusion molding or blow molding, sags and runs of resin under its own weight would easily occur, and this makes it difficult to carry out molding of large-scale products.
The melting property of a polycarbonate resin can be represented by the formula “Q=K·PN”, wherein “Q” represents an outflow rate of a molten resin (ml/sec), “K” represents a segment of the regression formula which is an independent variable derived from a molecular weight and/or structure of the polycarbonate resin, “P” represents a pressure value measured by a constant-load orifice-type flow tester at 280° C. (load: 10-160 kgf) (kg/cm2), and “N”-value represents a structural viscosity index.
When “N”=1 in the above formula, the resin exhibits a Newtonian fluid behavior. As N-value increases, the pressure dependency of flowability would increase and it would tend to exhibit a non-Newtonian fluid behavior.
The melt flow properties of polycarbonate resins used for large-volume hollow-molded products, large-scale extrusion-molded products and the like are evaluated by the above-mentioned N-value. In general, the resins exhibiting a non-Newtonian fluid behavior having a high pressure dependency of flowability is rather preferable, because sags and runs of resin or a drawdown at the time of extrusion and/or molding can be avoided.
Therefore, it is desired to produce arbitrarily a polycarbonate resin having such a preferred melt-flow property that the N-value thereof falls within the adequate range.
Therefore, according to the interfacial polymerization, in general, the non-Newtonian property when melted is controlled by methods such as adding a polycarbonate resin component having an extremely high molecular weight and taking a branching agent into a molecule voluntarily to form a branched structure. That is, blow moldability, drip preventing properties, flame retardance or the like are improved by increasing melt viscosity and/or elongation viscosity in a low shear rate region voluntarily.
These improvements are allowed because, according to the interfacial polymerization, there is a correlative relationship between the amount of a branching agent used and a degree of branching and the desired degree of branching can be adjusted arbitrarily by controlling the amount of the branching agent used.
According to the interfacial polymerization, however, toxic phosgene has to be used in the manufacturing method.
In addition, it remains a problem such as corrosion of equipment caused by by-products such as hydrogen chloride and sodium chloride and chlorine-containing compounds such as methylene chloride used in great quantities as a solvent, and difficulties in removal of impurities such as sodium chloride or residual methylene chloride which might have an influence on polymer properties.
Meanwhile, a melt-polymerization method which has been long known as another method for producing a polycarbonate resin is a method for producing polycarbonate from an aromatic dihydroxy compound and diarylcarbonate, wherein, for example, bisphenol A (BPA) and diphenylcarbonate (DPC) are polymerized through a transesterification reaction under melting conditions while removing by-product aromatic monohydroxy compounds away.
Unlike the interfacial polymerization method, the melt-polymerization method has advantages such as not using solvents. However, since the aromatic monohydroxy compound and diester carbonate in the high-viscosity molten polycarbonate should be removed during the manufacturing process, it is necessary to carry out a prolonged reaction under the conditions of high temperature and high vacuum. As a result, particular kind of equipment durable to a long term reaction at a high temperature under high vacuum and strong stirring devices to deal with a product having high viscosity are required as manufacturing equipment.
Regarding a high-molecular-weight polycarbonate produced by a conventional transesterification method, as shown in Non-Patent Documents 1 to 3, the degree of branching of the polycarbonate is unpredictable when molten since an unspecified amount of branched structures is generated during the manufacturing process. In addition, the polymer exhibits a large non-Newtonian property compared with a polymer produced by interfacial polymerization. As shown in Patent Documents 4-5, the branched structure thereof is caused by branching and/or cross-linking by ester bonding produced by subjecting polycarbonate to a reaction similar to the Kolbe-Schmitt reaction under the action of alkali, and it is known that controlling the amount of branched structures is difficult. That is, the amount of branched structure can be increased and decreased depending on the equipment used and operating conditions. It is extremely difficult to adjust the fluid behavior of polycarbonate when molten in accordance with various molding methods.
Moreover, a high-molecular-weight polycarbonate produced by a conventional transesterification method tends to be deteriorated in color tone and industrially only a yellowish polymer is obtained. Furthermore, it is known that the polymer obtained has a defect of low strength and is easy to cause brittle fracture.
Examples of conventional methods for solving the problem of color tone deterioration include an approach of shortening of time for the reaction by increasing the polymerization rate. More precisely, the molar ratio of DPCs/BPAs at the time of feeding for the polymerization reaction is adjusted to obtain the maximum polymerization rate stoichiometrically. The feeding ratio, which may also be influenced by the characteristics of polymerization reaction equipment, can be set at the range from 1.03 to 1.10, whereby relatively high polymerization rate can be obtained.
According to this method, though it may be effective in the low-molecular-weight range, since the polymerization reaction product becomes a fluid having an extremely high viscosity and the polymerization rate becomes extremely low in the high-molecular-weight range, deterioration of resin such as crosslinking and/or branching and deterioration in color tone caused by prolonged heat retention during polymerization or the like would be observed remarkably. Therefore, it was extremely difficult to obtain a high-molecular-weight polycarbonate wherein the desired amount of branched structures is adjusted arbitrarily by controlling the molar ratio of raw materials fed for polymerization. That is, in the case of producing a polycarbonate resin using a melt polymerization method, it was extremely difficult to quantitatively improve blow moldability, drip preventing properties, flame retardance or the like only by controlling melt viscosity and/or elongation viscosity in a low shear rate region and by controlling the added amount of a branching agent by in the same manner as in interfacial polymerization.
As a method for structural improvement of polycarbonate, there is an approach to decrease branched structures which occur naturally by a transesterification method for producing polycarbonate. For example, Patent Documents 1 and 2 propose a polycarbonate resin produced by a transesterification method which has no branching structure or has branching structures as little as possible. Patent Document 3 proposes a method for producing polycarbonate having 300 ppm or less of a Kolbe-Schmitt type branched structure.
Patent Documents 4 and 5 propose methods to improve color tone by preventing generation of branching structures caused by a side reaction which is extremely difficult in controlling by using a specific catalyst, and to introduce a specific branching structure positively by using a multi-functional compound. They disclose polycarbonate produced by a transesterification method wherein hollow-moldability is improved by increasing a non-Newtonian property of flow behavior.
However, these methods would not be common because they need using a particular kind of compound as a catalyst or a combination or selection of specific catalysts. Furthermore, when using polycarbonate thus obtained, harmful effects of catalysts on a human body and the environment are concerned.
Patent Document 6 discloses an attempt to improve mold flowability using 5-(dimethyl-p-hydroxybenzyl)salicylic acid as a branching agent. However, using this multi-functional compound has a problem which would easily cause generation of gel by a cross-linking action. Patent Documents 7 and 8 propose to adjust the amount of a Kolbe-Schmitt type branching structure derived from heat deterioration within a certain range by employing a particular kind of equipment, specific temperature range and retention conditions. However, according to this method, it is difficult to inhibit natural generation of branched structures fundamentally. Moreover, since the branched structure is a different kind structure generated naturally by heat deterioration reaction, it is necessary to use a particular kind of equipment under specific conditions of operation, in order to control the amount of branching structure as desired.
Patent Document 9 discloses a method of using acid anhydride as a branching agent. However, acid generation in the manufacturing process and the influences of introduction of ester bonding on properties and color tone cannot be ignored. Patent Document 10 discloses polycarbonate obtained by using a branching agent which has 1.36 or higher of a structural viscosity index. However, the relationship between the amount of a branching agent and a degree of branching cannot be found.
Accordingly, it is desired to develop an improved process for producing polycarbonate easily through a common transesterification method which enables to obtain polycarbonate having excellent color tone and physical properties wherein fluid behavior, non-Newtonian properties and molding flowability are well controlled as well as in polycarbonate obtained by interfacial polymerization, or to develop an improved process for producing polycarbonate through a transesterification method which enables to easily obtain polycarbonate having a desired degree of branching by controlling the degree of branching arbitrarily.
As an improvement of a process, Patent Document 11 discloses a manufacturing method using a particular kind of a horizontal stirring polymerization reactor as a final polymerization reactor. Patent Documents 12 and 12 disclose a method using a biaxial vent-type extruder. However, these methods are intended to promote elimination of phenol. Though high-molecular-weight polycarbonate may be obtained by this method, polycarbonate satisfying both physical properties and molding flowability cannot be obtained.
According to conventional method for producing high-molecular-weight aromatic polycarbonate, as mentioned above, various problems are remained to be solved in order to stably control branching structures as desired.
The present inventors had proposed a novel method for producing a high-molecular-weight aromatic polycarbonate resin which enables the increase in molecular weight of the aromatic polycarbonate resin satisfactorily while keeping good quality (Patent Document 14). According to the method, an aromatic polycarbonate prepolymer and a specific aliphatic diol compound as a linking agent are subjected to a transesterification reaction or a copolymerization reaction under reduced pressure in the presence of a transesterification catalyst to be linked with each other and to be highly polymerized. Thereby, a sufficiently highly polymerized polycarbonate resin having excellent properties that a polycarbonate originally has can be obtained.
The practical reaction scheme of the linking and highly-polymerizing reaction by the aliphatic diol compound is exemplified as follows:

According to the method, a highly polymerized aromatic polycarbonate resin having weight average molecular weight (Mw) of 30,000 to 100,000 can be produced in a short time by chain extension by linking the capped end of an aromatic polycarbonate with an aliphatic diol compound. According to the method, since polycarbonate is produced by a high-rate polymerization reaction, branching and/or cross-linking reactions caused by a prolonged heat retention can be inhibited, and thus, deterioration of polymer such as color change can be avoided.
Patent Documents 15 and 16 disclose a method for producing polycarbonate by adding divalent diols at latter stage of the transesterification reaction. However, they do not teach whether the degree of branching can be adjusted by controlling the additive amount of branching agents. Moreover, the polycarbonate thus obtained is not successfully satisfied in quality.
Thus, it is expected to develop a method for easily producing polycarbonate having a desired degree of branching by applying the above-mentioned technology of highly polymerizing which enables to obtain a sufficiently highly polymerized polycarbonate resin while keeping excellent properties that a polycarbonate originally has.